1. Field of the Invention
The present invention relates to a data communication apparatus, which performs communications of procedure signals at a low transmission speed, and performs communications of data at a high transmission speed.
2. Related Background Art
As a conventional apparatus of this type, for example, a facsimile apparatus of the G3 standards of the CCITT recommendation is known. The G3 facsimile apparatus performs a facsimile communication procedure at a transmission speed of 300 bps using a modem complying with the CCITT recommendation V21 standards, while it performs communications of image information at a transmission speed of 2,400 bps to 9,600 bps using a modem complying with the V27ter or V29 standards. The G3 facsimile apparatus performs communications while switching the transmission speed between high- and low-speed modes. As shown in FIGS. 5 and 6, the G3 facsimile apparatus has timings (to be referred to as nodes F hereinafter) at which high- and low-speed signals may be received. FIG. 5 shows a normal procedure, wherein a sender sends a PIX signal (V29 or V27ter) as an image signal in response to a CFR signal (V21), which indicates the end of reception preparation at a receiver. FIG. 6 shows a case wherein the sender sends a DCS signal for requesting a re-send mode since it failed to receive the CFR signal sent from the receiver.
Along with development of various functions, modems used in recent facsimile apparatuses incorporate all the modem modes such as V29, V27ter, V21, and the like required for facsimile communications. Some of these modems have a function of informing the presence of a signal energy, and a function of informing reception of a training signal of V29 or V27ter signals in the V29 or V27ter mode. When such a modem is used, a technique for, when a training signal cannot be received within a predetermined period of time (about 500 ms) after an ON timing of the signal energy, determining a low-speed signal in the V21 mode is used (since a V21 preamble signal continues for 1 sec in the facsimile procedure signals, reception can be made even when discrimination requires a long period of time). As a facsimile apparatus using such a technique, one disclosed in U.S. Pat. No. 4,910,506 is known.
Some other modems have a function of informing detection of a V21 mode signal in the V29 or V27ter mode to the control side. A modem of this type stands by in the V29 or V27ter mode, and re-sets the modem mode to the V21 mode upon reception of information indicating detection of the V21 mode signal so as to receive the V21 signals (in the modem of this type, the detection capability of the V21 signals is very important).
In the above-mentioned two prior arts, in a line such as an international line suffering from an echo, since the V21 mode signal is actually returned as an echo at a T30 node F, the control exits the high-speed V29 mode, and starts reception of V21 signals. Even in such a case, when the echo delay time is long, since a CFR signal returned as an echo can be received, as shown in FIG. 7, it can be recognized as an echo of its own station signal. However, the echo delay time varies depending on a communication channel at that time. Therefore, when the delay time is short, the V21 mode signal cannot be recognized as an echo of its own station signal, as shown in FIG. 8, and the control remains switched to the V21 mode. As a result, the training signal of a PIX signal (V29) cannot be received.